Flat lapping of workpiece surfaces to produce precision-flat and mirror smooth polished surfaces at high production rates where the opposing workpiece surfaces are co-planar is required for many high-value parts such as semiconductor wafer and rotary seals. The accuracy of the lapping or abrading process is constantly increased as the workpiece performance, or process requirements, become more demanding. The new workpiece feature tolerances for flatness accuracy, the amount of material removed, the absolute part-thickness and the smoothness of the polish become more progressively more difficult to achieve with existing abrading machines and abrading processes. In addition, it is necessary to reduce the processing costs without sacrificing performance. Also, it is highly desirable to eliminate the use of messy abrasive slurries. Changing the abrading process set-up of most of the present abrading systems to accommodate different sized abrasive particles, different abrasive materials or to match abrasive disk features or the size of the abrasive disks to the workpiece sizes is typically tedious and difficult.
This invention references commonly assigned U.S. Pat. Nos. 5,910,041; 5,967,882; 5,993,298; 6,048,254; 6,102,777; 6,120,352; 6,149,506; 6,607,157; 6,752,700; 6,769,969; 7,632,434 and 7,520,800 and commonly assigned U.S. patent application published numbers 20100003904; 20080299875 and 20050118939 and all contents of which are incorporated herein by reference.
There are many different types of abrading and lapping machines that have evolved over the years. Slurry lapping has been the primary method of providing precision-flat and smoothly polished flat-surfaced workpieces using a liquid mixture of loose abrasive particles that is applied to a flat surfaced rotary platen that is pressed into contact with the rotating workpieces. The platen surface continually wears due to abrading contact with the workpieces and conditioning rings are used periodically or continuously to re-establish the required planar flatness of the platen. Most slurry lapping is single-sided where only the exposed surface of a workpiece is abraded. Double-sided slurry lapping can be done by using two abrading platens that mutually contact both surfaces of the flat workpieces that are sandwiched between the two rotating abrading platens. The upper platen floats to allow conformal contact with the workpieces that are placed in flat contact with the flat surface of the lower platen. Workpieces are rotated with the use of gear-driven planetary workholders where it is required that the workholders geared-disks are thinner than the workpieces. Slurry lapping typically uses low abrading pressure and it is slow and messy. Changing the size of abrasive particles requires that the messy platens have to be thoroughly cleaned before smaller-sized particles are used because a few straggler-type large-sized particles can result in scratches of high-value workpiece surfaces. Abrading processes require that the abrasive sizes be sequentially changed (typically in three steps) to minimize the time required to flatten and polish the surfaces of workpieces.
Micro-grinding (flat-honing) is a double-sided abrading process that uses two abrading platens that mutually contact both surfaces of the flat workpieces that are sandwiched between the two rotating abrading platens. Both the upper and lower platen annular abrading surfaces have a thick layer of fixed-abrasive materials that are bonded to abrasive-wheels, where the abrasive wheels are bolted to the platen surfaces. The upper platen floats to allow conformal contact with the workpieces that are placed in flat contact with the flat surface of the lower platen. Workpieces are rotated with the use of gear-driven planetary workholders where it is required that the workholders geared-disks are thinner than the workpieces. Micro-grinding is slow and very high abrading pressures are typically used. Changing the abrasive wheels is a time-consuming and complex operation so the abrasive wheels are typically operated for long periods of time before changing. Changing the size of abrasive particles requires that the abrasive wheels have to be changed.
Chemical mechanical planarization (CMP) of workpieces typically use a resilient flat-surfaced pad that is coated with a continuous or periodic flow of liquid slurry that contains loose abrasive particles and specialty chemicals that enhance the abrading characteristics of select workpiece materials. Flat-surfaced workpieces are placed in flat contact with the rotating pads where the workpieces are also typically rotated. The pads often have fiber construction where it has been estimated that only 10% of the individual fiber strands are in abrading contact with the workpiece surface as the workpiece is forced into the surface-depth of the resilient pads. It also has been estimated that 30% of the expensive diamond or other abrasive particles are lost before being utilized for abrading contact with the workpieces. As in slurry lapping, this CMP polishing process is messy. Changing the size of the abrasive particles requires that a new or different pad is used with the new-sized particles. Because the workpieces float on the surface of the resilient pads, the CMP process is a polishing process only. Very small surface protuberances are removed from the flat surfaces of semiconductor wafers but the precision flatness of a wafer can not be established by the CMP process because of the floatation of the wafers on the pad surface.
More recently, fixed-abrasive web material is used for CMP polishing of wafers. The web has shallow-height islands that are attached to a web backing and the abrasive web is incrementally advanced between times of polishing individual wafers held in flat contact with the stationary web. Water containing chemicals is applied to the wafers during the polishing procedure. The abrasive web is typically supported by a semi-rigid polymer surface that is supported by a resilient pad. When the abrasive web is stationary, the wafer is rotated. However, the rotated wafer has a near-zero abrading speed at the rotated wafer center. Because the well-established function of the workpiece material removal rate being directly proportional to the abrading speed, the material removal rate is very high at the outer periphery of the rotating wafer but near-zero at the wafer center. This results in non-uniform abrading of the wafer surface. The fixed-abrasive provides a clean CMP abrading process compared to the messy slurry-pad CMP process.
U.S. Pat. No. 7,614,939 (Tolles et al) describes a CMP polishing machine that uses flexible pads where a conditioner device is used to maintain the abrading characteristic of the pad. Multiple CMP pad stations are used where each station has different sized abrasive particles. U.S. Pat. No. 4,593,495 (Kawakami et al) describes an abrading apparatus that uses planetary workholders. U.S. Pat. No. 4,918,870 (Torbert et al) describes a CMP wafer polishing apparatus where wafers are attached to wafer carriers using vacuum, wax and surface tension using wafer. U.S. Pat. No. 5,205,082 (Shendon et al) describes a CMP wafer polishing apparatus that uses a floating retainer ring. U.S. Pat. No. 6,506,105 (Kajiwara et al) describes a CMP wafer polishing apparatus that uses a CMP with a separate retaining ring and wafer pressure control to minimize over-polishing of wafer peripheral edges. U.S. Pat. No. 6,371,838 (Holzapfel) describes a CMP wafer polishing apparatus that has multiple wafer heads and pad conditioners where the wafers contact a pad attached to a rotating platen. U.S. Pat. No. 6,398,906 (Kobayashi et al) describes a wafer transfer and wafer polishing apparatus. U.S. Pat. No. 7,357,699 (Togawa et al) describes a wafer holding and polishing apparatus and where excessive rounding and polishing of the peripheral edge of wafers occurs. U.S. Pat. No. 7,276,446 (Robinson et al) describes a web-type fixed-abrasive CMP wafer polishing apparatus.
U.S. Pat. No. 6,786,810 (Muilenberg et al) describes a web-type fixed-abrasive CMP article. U.S. Pat. No. 5,014,486 (Ravipati et al) and U.S. Pat. No. 5,863,306 (Wei et al) describe a web-type fixed-abrasive article having shallow-islands of abrasive coated on a web backing using a rotogravure roll to deposit the abrasive islands on the web backing. U.S. Pat. No. 5,314,513 (Milleret al) describes the use of ceria for abrading.
Various abrading machines and abrading processes are described in U.S. Pat. Nos. 1,989,074 (Bullard), 2,410,752 (Sells et al), 2,696,067 (Leach), 2,973,605 (Carman et al), 2,979,868 (Emeis), 3,342,652 (Reisman et al), 3,475,867 (Walsh), 3,662,498 (Caspers), 4,104,099 (Scherrer), 4,165,584 (Scherrer), 4,256,535 (Banks), 4,315,383 (Day), 4,588,473 (Hisatomi et al), 4,720,938 (Gosis), 4,735,679 (Lasky), 4,910,155 (Cote et al), 5,032,544 (Ito et al), 5,137,542 (Buchanan et al), 5,191,738 (Nakazato et al), 5,274,960 (Karlsrud), 5,364,655 (Nakamura et al), 5,422,316 (Desai et al), 5,454,844 (Hibbard et al), 5,456,627 (Jackson et al), 5,538,460 (Onodera), 5,569,062 (Karlsrud), 5,643,067 (Katsuoka et al), 5,769,697 (Nisho), 5,800,254 (Motley et al), 5,833,519 (Moore), 5,840,629 (Carpio), 5,857,898 (Hiyama et al), 5,860,847 (Sakurai et al), 5,882,245 (Popovich et al), 5,916,009 (Izumi et al), 5,938,506 (Fruitman et al), 5,964,651 (Hose), 5,972,792 (Hudson), 5,975,997 (Minami), 5,981,454 (Small), 5,989,104 (Kim et al), 5,916,009 (Izumi et al), 6,007,407 (Rutherford et al), 6,022,266 (Bullard et al), 6,089,959 (Nagahashi), 6,139,428 (Drill et al), 6,165,056 (Hayashi et al), 6,168,506 (McJunken), 6,217,433 (Herrman et al), 6,273,786 (Chopra et al), 6,439,965 (Ichino), 6,520,833 (Saldana et al), 6,632,127 (Zimmer et al), 6,652,764 (Blalock), 6,702,866 (Kamboj), 6,893,332 (Castor), 6,896,584 (Perlov et al), 6,899,603 (Homma et al), 6,935,013 (Markevitch et al), 7,001,251 (Doan et al), 7,008,303 (White et al), 7,014,535 (Custer et al), 7,029,380 (Horiguchi et al), 7,033,251 (Elledge), 7,044,838 (Maloney et al), 7,125,313 (Zelenski et al), 7,144,304 (Moore), 7,147,541 (Nagayama et al), 7,166,016 (Chen), 7,214,125 (Sharples et al), 7,250,368 (Kida et al), 7,364,495 (Tominaga et al), 7,367,867 (Boller), 7,393,790 (Britt et al), 7,422,634 (Powell et al), 7,446,018 (Brogan et al), 7,449,124 (Webb et al), 7,456, 7,527,722 (Sharan), 7,582,221 (Netsu et al), 7,585,425 (Ward), 7,588,674 (Frodis et al), 7,635,291 (Muldowney), 7,648,409 (Horiguchi et al) and in U.S. Patent Application 2008/0182413 (Menk et al).
I. Types of Abrading Contact
The characteristic of workpieces abrasion is highly dependent on the type of contact that is made with an abrasive surface. In one case, the flat (or curved) surface of a rigid platen-type surface is precisely duplicated on a workpiece. This is done by coating the platen with abrasive particles and rubbing the workpiece against the platen. In another case, a rigid moving abrasive surface is guided along a fixed path to abrade the surface of a workpiece. The accuracy of the abrasive guide-rail (or a rotary spindle) determines the accuracy of the abraded workpiece surface. A further case is where workpieces are “floated” in conforming surface-contact with a moving rigid abrasive-coated flat platen. Here, only the high-spot areas of the moving platen contact the workpiece. It is helpful that the abraded surface of the workpiece is typically flatter than the abrading surface of the platen.
For those workpieces requiring ultra-flat surfaces where the amount of material removed in an abrading process is extremely small, it is difficult to provide fixed-path abrading machines having rigid abrasive surfaces that can accomplish this. Out-of-plane variations of the moving abrasive are directly dependent on the variations of the moving abrading machine components. Abrading machines typically are not capable of providing moving abrading surfaces that have variations less than the often-required 1 lightband (0.000011 inches or 11 millionths of an inch) of workpiece flatness. It is much more difficult to create precision-flat and mirror-smooth surfaces on large sized workpieces than small ones.
Most lapping-type of abrading is done on rotary-platen machines that provide smooth continuous abrading motion rather than oscillating-motion machines. However, rotary-motion machines have an inherent flaw in that the abrading speed is high at the outer periphery of the platen and low at the platen center. This change of abrading speed across the surface of the platen results in non-uniform abrading of a workpiece surface. Using annular bands of abrasive on large diameter platens minimizes this problem. However, it is necessary to rotate workpieces while in abrading contact with the platen abrasive to even-out the wear on a workpiece.
Wear-down of the platen abrasives during abrading creates non-flat abrasive surfaces which prevent abrading precision-flat workpiece surfaces. It is necessary to periodically re-flatten the platen abrading surfaces.
For removing small amounts of surface material for workpieces, floatation-type abrading systems are often used. Here, conformal abrading contact provides uniform material removal across the full flat surface of a workpiece. One common-use of floatation-abrading is slurry lapping. Here, a flat platen is surface-coated with a liquid slurry mixture of abrasive particles and a workpiece is held in flat conformal contact with the slurry coated platen. This slurry lapping system can provide workpieces having both precision-flatness across the full workpiece surface and a mirror-smooth polish.
Another abrading system that has “floatation” characteristics is double-sided abrading. Here, equal-thickness workpiece parts are position around the circumference of a lower flat-surfaced abrasive platen. Then another flat-surfaced abrasive platen is placed in conformal contact with the top surface of the distributed workpieces. This upper abrasive platen is allowed to “float” while both abrasive platens are moved relative to the workpieces sandwiched between them.
II. Single-Sided Abrading
Abrading ultra-flat and ultra-smooth workpiece parts requires a sequential series of different abrading techniques. First, rigid-grind techniques are used. Here the, rough-surfaced workpieces are given flat surfaces that are fairly smooth. Then, workpieces are lapped even flatter and smoother. Precision-flat rigid platens are coated with a slurry containing loose abrasive particles are used for lapping. This slurry lapping process can produce workpieces that are much flatter than the platen surfaces. This is a critical achievement because it is not possible to produce and maintain platens that have surfaces that are as desired flatness of the workpieces.
Likewise, it is not possible to provide and maintain lapping machines that rotating workholders that are perfectly perpendicular to a rotary abrasive platen surface. Because of the lack of machine capability, it is not practical to produce workpieces having precisely parallel surfaces using this type of single-sided abrading machines.
III. Double-Sided Abrading
To produce parallel-surfaced workpieces, a different machine technology is used. Here, a large-diameter rigid precision-flat rotating platen is provided. Multiple equal-thickness workpieces are positioned around the circumference of the platen. Then, another large diameter flat-surfaced abrading platen is placed in contact with the top surfaces of the multiple workpieces. Here, the upper platen is allowed to float spherically so its flat surface assumes parallelism with the surface of the bottom platen. Both the upper and bottom platens have equal-diameter abrading surfaces. With this technology, no attempt is made to rigidly position the surface of the upper moving abrasive platen surface precisely perpendicular to the surface of the bottom platen. This co-planar alignment of the two double-sided abrading platens is achieved with ease and simplicity by using the uniform-thickness workpieces as spacers between the two [platens.
Building of complex and expensive rigid-workholder style of machines to abrade precisely co-planar (parallel) workpiece surfaces is avoided by this technique of double-sided abrading. The simple, and less expensive, machines provide an upper platen that floats spherically while rotationally moving in abrading contact with the top surface of the workpieces. Because both workpieces are abraded simultaneously, the workpiece surfaces are precisely co-planar.
IV. CMP Slurry Abrading of Wafers
Floatation-type abrading machines are typically used for abrading workpieces requiring ultra-flat and ultra-smooth workpiece surfaces. For example, high-value semiconductor wafers are constructed from a combination of rigid silicon materials and soft metals. They are often very thin and fragile but have ultra-flat and smooth-polish requirements. Another type of flotation-abrading is used to abrade these wafers after each sequential depositions of material upon the wafer surfaces. This chemical mechanical planarization (CMP) system uses resilient pads that are coated with a liquid slurry mixture containing loose-abrasive particles. Rotating wafers are held in flat abrading contact with the flat moving pad surface. This is considered a “floating” abrading system. Here, the wafers are “plunged” into the surface-depths of the resilient pad where conformal full-surface contact of the wafer is made with the pad surface.
Fixed-abrasive CMP abrading of wafers is also done using thin flexible backings that are coated with shallow-height abrasive islands. These island-backing articles are supported by semi-rigid plates that “float” on a resilient foam pad. The abrasive island backing articles are held stationary while the wafers are rotated while in full-faced contact with the abrasive.
Sequential polishing of semiconductor wafers after each deposition of new materials on the wafer surface requires a completely different abrading technology. The material deposition layers are extremely thin and the wafers are very large in size. It is not possible to construct abrading machines having rigid workholder and rigid abrasive surfaces to remove protrusions (only) from the ultra-thin deposition layers. Instead, a completely different abrading approach is used. First, the wafers are ground or lapped precisely flat. Then, a material layer is deposited on the wafer. A chemical mechanical planarization (CMP) planarization process is used to remove only the unwanted protrusions of this deposited material. Here, the wafer is held face-down, under low pressure, against a non-rigid, abrasive slurry coated resilient foam disk pad. The resilient foam pad provides conformal contact of the pad surface with the flat wafer surface. The pad disk rotates and the workpiece is also rotated to provide abrading speed across the whole surface wafer surface. Loose-abrasive soft ceria particles are mixed in the liquid slurry applied to the pad surface. The pH of the slurry liquid is elevated to soften the surface of the applied wafer deposition material. Abrading the undesired softened protrusions is a very gentile action compared with conventional hard abrasive particle abrading action.
No planarization attempt is made to correct any global non-flat regions of the whole wafer surfaces. Only localized planarization is provided where only individual protrusions are removed.
V. Fixed-Abrasive CMP Wafer Abrading
Fixed-abrasive media is now being used for CMP abrading of wafers. Here, there is no liquid abrasive slurry mess because the abrasive particles are bonded in shallow-height islands on a flexible backing sheet. This fixed abrasive media is in a web-roll form. Sections of the abrasive web are stretched over a semi-rigid flat-surfaced polymer platen. The rigid platen is supported by a resilient foam-type pad. Abrading speed is provided by rotating the wafer while it is in full-face contact with the stationary raised-island abrasive. The abrasive is not moved relative to the wafer. This fixed-abrasive system is different than the abrasive slurry CMP system where relative abrading speed is provided by a moving slurry pad. Water having elevated pH is applied to the abrasive surface.
VI. Raised-Island High Speed Flat Lapping
All of the present precision-flat abrading processes have very slow abrading speeds of about 5 mph. The high speed flat lapping system operates at about 100 mph. Increasing abrading speeds increase the material removal rates. This results in high workpiece production and large cost savings. In addition, those abrading processes that use liquid abrasive slurries are very messy. The fixed-abrasive used in high speed flat lapping eliminates the slurry mess. Another advantage is the quick-change features of the high speed lapper system where abrasive disks can be quickly changed with use of the disk vacuum attachment system. Changing the sized of the abrasive particles on all of the other abrading systems is slow and troublesome. The precision-thickness raised island abrasive disks that are used in high speed flat lapping can also be used for CMP-type abrading, but at lower speeds. These disks can be provided with thick semi-rigid backings that are supported with resilient foam backings.
VII. Abrading Platens
A. Rotary Platens
Rotary platens are used for lapping because it is easy to establish and maintain their moving precision-flat surfaces that support abrasive coatings. The flat abrasive surfaces are replicated on workpieces where non-flat abrasive surfaces result in non-flat workpiece surfaces. Rotary platens also provide the required continuous smooth abrading motion during the lapping operation because they don't reverse direction as does an oscillating system. However, the circular rotary platen annular abrasive bands are curved which means the outer periphery travels faster than the inner periphery. As a result, the material cut-rate is higher at the outside portion of the annular band than the inside. To minimize this radial position cut rate disparity, very large diameter platens are used to accommodate large workpieces.
B. Maintain Abrasive Surface Flatness
To provide precision-flat workpiece surfaces, it is important to maintain the required flatness of annular band of fixed-abrasive coated raised islands during the full abrading life of an abrasive disk. The techniques developed to maintain the abrasive surface flatness are very effective. The primary technique is to use the abraded workpieces themselves to keep the abrasive flat during the lapping process. Here large workpieces (or small workpieces grouped together) are also rotated as they span the radial width of the rotating abrasive band. Another technique uses driven planetary workholders that move workpieces in constant orbital spiral path motions across the abrasive band width. Other techniques include the use of annular abrasive coated conditioning rings. These rings can rotate in stationary positions or be transported by planetary circulation mechanisms. Conditioning rings have been used for years to maintain the flatness of slurry platens that utilize loose abrasive particles. These same types of conditioning rings are also used to periodically re-flatten the fixed-abrasive continuous coated platens used in micro-grinding.
C. No Platen Wear
Unlike slurry lapping, there is no abrasive wear of raised island abrasive disk platens because only the non-abrasive flexible disk backing surface contacts the platen surface. There is no motion of the abrasive disk relative to the platen because the disk is attached to the platen. During lapping, only the top surface of the disk raised island fixed-abrasive has to be kept flat, not the platen surface itself. Here, the precision flatness of the high speed flat lapper system can be completely re-established by simply and quickly changing the abrasive disk. Changing the non-flat fixed abrasive surface of a micro-grinder can not be done quickly because it is a bolted-on integral part of the rotating platen that supports it.
D. Quick-Change Capability
Vacuum is used to quickly attach flexible abrasive disks, having different sized particles, different abrasive materials and different array patterns and styles of raised islands. Each flexible disk conforms to the precision-flat platen surface provide precision-flat planar abrading surfaces. Quick lapping process set-up changes can be made to process a wide variety of workpieces having different materials and shapes with application-selected raised island abrasive disks that are optimized for them individually. Small and medium diameter disks can be stored or shipped flat in layers. Large and very large disks can be rolled and stored or shipped in polymer protective tubes. The abrasive disk quick change capability is especially desirable for laboratory lapping machines but they are also great for prototype lapping and full-scale production lapping machines. This abrasive disk quick-change capability also provides a large advantage over micro-grinding where it is necessary to change-out a worn heavy rigid platen or to replace it with one having different sized particles.
VIII. Hydroplaning of Workpieces
Hydroplaning of workpieces occurs when smooth surfaces (continuous thin-coated abrasive) are in fast-moving contact with a flat surface in the presence of surface water. However, it does not occur when interrupted-surfaces (raised islands) contact a flat wetted workpiece surface. An analogy is the tread lugs on auto tires which are used on rain slicked roads. Tires with lugs grip the road at high speeds while bald smooth-surfaced tires hydroplane.
IX. Maintaining Abrasive Disk Flat Surface
Care is taken during the lapping procedures to maintain the precision flatness of the abrasive surface. This is done by selecting abrasive disks where the full surface of the abrasive is contacted by the workpiece surface. This results in uniform wear-down of the abrasive. Other techniques can also be used to accomplish this. First, a workpiece that is smaller than the radial width of the annular band of abrasive islands can be oscillated radially during the abrading procedure to overlap both the inner and outer edges of the annular abrasive band. This prevents the formation of tangential raised ribs of abrasive inboard and outboard of the wear-track of the workpiece.
Also, stationary-position conditioning rings can be used in flat contact with the moving abrasive. These rings have diameters that are larger than the radial width of the abrasive island annular band. They preferentially remove the undesirable raised abrasive high spot areas or even raised rib-walls of abrasive that extend around the circumference of the annular band of abrasive. The conditioning rings are similar to those used in slurry lapping to continually maintain the flatness of the rotating slurry platen.
Many of the different techniques used here to maintain the flatness of annular band of fixed-abrasive coated raised islands during the abrading life of an abrasive disk are highly developed and in common use in slurry lapping. In slurry lapping, a liquid mixture that contains loose abrasive particles continuously wears recessed circumferential tracks in the rigid metal platen surface. However, unlike slurry lapping, there is no abrasive wear of the high speed flat lapper platens because only the flexible disk backing contacts the platen surface. Here, the precision flatness of the high speed flat lapper system is re-established by simply changing the abrasive disk.
Another method of maintaining the planar flatness of both the upper and lower abrasive platens used in double-sided lapping is to translate the upper platen radially relative to the lower platen during the recondition process. Instead of the upper and lower platens being held in a concentric position during the flatness reconditioning process, the upper platen is moved to where they are not concentric. The amount of radial motion required is limited because the radial width of the annular band of abrasive is small relative to the platen diameters. Radial off-setting of the platens takes place but the floating upper platen is still allowed to maintain its flat conformal contact with the lower platen surface. Abrading mutually takes place on both abrasive platen surfaces as both the platens are rotated. This platen surface abrading action allows abrasive from one platen to travel cross-width relative to the abrasive on the opposing platen.
Off-set abrading action prevents tangential out-of-plane faults on one platen abrasive surface being transferred to the abrading surface of the opposite platen when the two platen surfaces are reconditioned while they are concentric. The upper platen off-set can be stationary or the upper platen can be oscillated relative to the lower platen during the reconditioning event.
Because the upper platen uses a spherical bearing that allows the platen to float, the platen holding mechanism can be a simple pivot arm device. The platen spherical-action bearing provides radial support for the platen during rotation so the platen retains its balance even when it is operated at great speeds. Conformal flat contact of the two platens prevents wobble of the upper platen as it is rotated. It is not necessary that the pivot arm position the upper platen in a precision concentric alignment with the lower platen during a double-sided lapping operation.
X. Raised Island Disks
The reason that this lapping system can be operated at such high speeds is due to the use of precision-thickness abrasive coated raised island disks. Moving abrasive disks are surface cooled with water to prevent overheating of both the workpiece and the abrasive particles. Raised islands prevent hydroplaning of the stationary workpieces that are in flat conformal contact with water wetted abrasive that moves at very high speeds. Abrading speeds are often in excess of 100 mph. Hydroplaning occurs with conventional non-island continuous-coated lapping film disks where a high pressure water film is developed in the gap between the flat workpiece and the flat abrasive surfaces.
During hydroplaning, the workpiece is pushed up away from the abrasive by the high pressure water and also, the workpiece is tilted. These cause undesirable non-flat workpiece surfaces. The non-flat workpieces are typically polished smooth because of the small size of the abrasive particles. However, flat-lapped workpieces require surfaces that are both precision-flat and smoothly polished.
The islands have an analogy in the tread lugs on auto tires which are used on rain slicked roads. Tires with lugs grip the road at high speeds while bald tires hydroplane. Conventional continuous-coated lapping film disks are analogous to the bald tires.
Raised islands also reduce “stiction” forces that tend to bond a flat surfaced workpiece to a water wetted flat-surfaced abrasive surface. High stiction forces require that large forces are applied to a workpiece when the contacting abrasive moves at great speeds relative to the stationary workpiece. These stiction forces tend to tilt the workpiece, resulting in non-flat workpiece surfaces. A direct analogy is the large attachment forces that exist between two water-wetted flat plates that are in conformal contact with each other. It is difficult to slide one plate relative to the other. Also, it is difficult to “pry” one plate away from the other. Raised island have recessed channel passageways between the island structures. The continuous film of coolant water that is attached to the workpiece is broken up by these island passageways. Breaking up the continuous water film substantially reduces the stiction.
XI. Precision Thickness Disks
Another reason that this lapping system can be operated at such high speeds is due to the use of precision-thickness abrasive coated raised island disks. These disks have an array of raised islands arranged in an annular band on a disk backing. To be successfully used for high speed lapping, the overall thickness of the abrasive disks, measured from the top surface of the exposed abrasive to the bottom mounting surface of the disk backing must be uniform across the full disk-abrasive surface with a standard deviation in thickness of less than 0.0001 inches. The top flat surfaces of the islands are coated with a very thin coating of abrasive. The abrasive coating consists of a monolayer of 0.002 inch beads that typically contain very small 3 micron (0.0001 inch) or sub-micron diamond abrasive particles. Raised island abrasive disks are attached with vacuum to ultra-flat platens that rotate at very high abrading surface speeds, often in excess of 100 mph.
The abrasive disks have to be of a uniform thickness over the full abrading surface of the disk for three primary reasons. The first reason is to present all of the disk abrasive in flat abrading contact with the flat workpiece surface. This is necessary to provide uniform abrading action over the full surface of the workpiece. If only localized “high spots” abrasive surfaces contact a workpiece, undesirable tracks or gouges will be abraded into the workpiece surface. The second reason is to allow all of the expensive diamond abrasive particles contained in the beads to be fully utilized. Again if only localized “high spots” abrasive surfaces contact a workpiece, those abrasive particles located in “low spots” will not contact the workpiece surface. Those abrasive beads that do not have abrading contact with a workpiece will not be utilized. Because the typical flatness of a lapped workpiece are measured in millionths of an inch, the allowable thickness variation of an raised island abrasive disk to provide uniform abrasive contact must also have extra-ordinary accuracy.
The third reason is to prevent fast moving uneven “high spot” abrasive surfaces from providing vibration excitation of the workpiece that “bump” the workpiece up and away from contact with the flat abrasive surface. Because the abrasive disks rotate at such high speeds and the workpieces are lightweight, these moving bumps tend to repetitively drive the workpiece up after which it falls down again with only occasional contact with the moving abrasive. The result is uneven wear of the workpiece surface.
All three of these reasons are unique to high speed flat lapping. The abrading problems, and solutions described here were progressively originated while developing this total lapping system. They were not known or addressed by others who had developed raised island abrasive disks. Because of that, their disks can not be used for high speed flat lapping.
XII. Abrading Pressure
Abrading pressures used are typically a small fraction of that used in traditional abrading processes. This is because of the extraordinary cutting rates of the diamond abrasive at the very high abrading speeds. Often abrading pressures of less than 0.2 psi can be used in high speed flat lapping. These low pressures have a very beneficial effect as they result in very small amounts of subsurface damage of workpiece materials that is typically caused by the abrasive material.
XIII. Annular Band of Abrasive
The raised abrasive islands are located only in an annular band that is positioned at the outer periphery of the disk. Problems associated with the uneven wear-down of abrasives located at the inner radius of a disk are minimized. Also, the uneven cutting rates of abrasives across the abrasive surface due to low abrading speeds at the innermost disk are minimized. Equalized cutting rates across the radial width of the annular band occur because the localized abrading speeds at the inner and outer radii of the annular abrasive band are equalized.
The abrasive islands are constructed in annular bands on a flexible backing. The disks are not produced from continuous abrasive coated webs is not used because the presence of abrasive material at the innermost locations on a disk are harmful to high speed flat lapping. In addition, there are no economic losses associated with the lack of utilization of expensive diamond particles located at the undesirable innermost radii of an abrasive disk.
XIV. Initial Platen Flatness
The best flatness that is practical to achieve for a new (or reconditioned) slurry platen having a medium platen diameter is about 0.0001 inches. It is even more difficult to achieve this flatness for large diameter platens. These are platen flatness accuracies that are achieved immediately after a platen is initially flattened. This process is usually done with great care and requires great skill and effort. To better appreciate the small size of this 0.0001 inch allowable platen variation, a human hair has a diameter of about 0.004 inches and a sheet of copier paper is also about 0.004 inches thick. Attaining a flatness variation of 0.0001 inches is difficult for a medium 12 inch diameter platen, more difficult for a large 6 foot platen and extremely difficult for huge platens that exceed 30 feet in diameter.
The vertical distance that a typical outer periphery deviates from the platen planar surface far exceeds the size of a submicron abrasive particle. To appreciate the relative difference between platen flatness deviation dimensions and the abrasive particle sizes, a comparison is made here. Typically a new (or reconditioned) platen is flattened to within 0.0001 inches total variation of the platen plane. This is roughly equivalent to the size of a 3 micron abrasive particle. It is also approximately equal to 10 helium lightbands of flatness. These dimensions are so small that optical refraction devices are used to measure flatness variations in lightbands. It is difficult to accurately make these small measurements using conventional mechanical measuring devices. The out-of-plane platen flatness is even worse when compared to sub-micron sized abrasive particles. For instance, a typical 0.3 micron particle is only one tenth the size of a 3 micron particle. Even the typical non-worn platen flatness variations are grossly larger than the size of the sub-micron particles that are required to produce mirror-smooth polishes.
XV. Continual Wear of Platen Surface
Even though a platen can initially have a precision-flat planar surface, this surface is constantly subjected to uneven wear. The platen uneven wear is caused primarily by the variation of the abrading speeds across the radial surface of the rotating platen. Abrading speeds are higher at the outer periphery of a circular rotating platen than they are at the inner radial location due to the greater circumference at the outer periphery. Higher abrading speeds mean higher wear. This results in continual higher wear of the platen at the outer periphery. The worn outer periphery area then develops an annular band that is lower than the plane of the overall platen surface. This out-of-plane platen wear is caused primarily by the loose abrasive particles, not the imbedded particles.
XVI. Platen Wear Effect on Workpiece
As a platen is subjected to uneven wear only the high-spot areas of a rotating platen are in abrading contact with a flat workpiece surface. More uneven platen wear means that uneven workpiece material removal becomes more pronounced.
XVII. Conditioning Rings
In addition, a conditioning ring can make flat abrading contact with the annular abrasive band to periodically dress the full radial and tangential surface of the abrasive band into a precision plane. These conditioning rings are the same as used for slurry lapping. For slurry lapping, they prevent abrading an annular groove in the rotating platen surface. For high speed raised island disks, they prevent abrading an annular groove in the planar abrasive surface.
XVIII. Raised Island Disk Features
A. Precision Thickness Abrasive Disks
The abrasive disks that are used to produce a flat lapped workpiece generally are used in sets of three. The first disk uses a coarse abrasive to initially flatten a rough surfaced workpiece. The second disk uses a medium abrasive to develop a smooth surface while retaining the flatness. The third disk has very small abrasive particles to generate the polished surface, again while retaining the surface flatness. The abrasive disks are used sequentially on the lapper machine and the sequence is repeated until the abrasive disks are worn out. Typical disks have very long lives because of the long life of the abrasive beads that are filled with diamond particles.
Because the flatness of a workpiece is directly related to the flatness of the abrasive disk, it is critical that new disks have a precision thickness across the full surface of the disk. Each disk must be manufactured with a uniform thickness across the surface of the abrasive islands that typically has a thickness variation that is less than 0.0001 inches to assure that the disk can be used satisfactorily to produce flat lapped workpiece parts. This disk thickness accuracy is required for the high speed abrasive disks used in this operation and is not available with traditional raised island abrasive disks.
One simple method to manufacture raised island abrasive disks that have the required disk thickness is to produce polymer disk backings that have annular bands patterns of raised island structures attached to the backing. Then the island top surfaces are ground to have the same precision height from the backside of the backing. A mixture of abrasive beads, a solvent and an adhesive provides a mixture that has a uniform distribution of the beads in the adhesive mixture. This mixture is applied to the top flat surface of the islands to form a monolayer of abrasive beads. After partial drying of the adhesive which tends to “skin-over”, the tops of the individual beads can be pressed into a common plane that is parallel to the backside of the disk backing. This assures that all the individual abrasive beads are utilized in the abrading procedures. Also, the abrasive disk now has a precision thickness across the whole abrasive surface of the abrasive. The nominal thickness of the abrasive disk is relatively unimportant as a a workpiece is simply lowered to contact the abrasive. It is primarily the precision thickness control of the disk that is important.
It is desirable that the inner diameter of the annular abrasive band is greater than approximately 50% of the outer diameter of the annular band to equalize the abrading surface speeds across the radial width of the band. Each high speed abrasive disk has an annular band of abrasive coated raised islands to provide abrading speeds that are approximately constant across the radial width of the annular abrasive. Typically, the width of the workpiece is approximately equal to the radial width of the annular abrasive band to assure that the abrasive is worn down evenly during the abrading process. When large workpieces are abraded, then the annular width of the abrasive disk has to be equally large.
The abrasive disks are flexible to conform to the flat surface of a rotary platen. The disk backing is typically made from a polymer sheet having a thickness of less than 0.005 inches. The bottom mounting surface of the backing is smooth and continuous to provide a vacuum seal when the disk is mounted to a flat platen. It is preferred that the disks are used on flat surfaced rotary platens.
B. Thickness Related to Disk Diameter
Small diameter abrasive disks having low-height raised islands can be moderately thin and use polymer backings. Large diameter disks require thicker backings for abrading durability and for handling and storage. Thick, but flexible disks are easier to attach to large diameter platens than are thin disks.
C. Thickness Related to Island Heights
Thicker backings are required for disks having raised island structures that protrude substantially from the top surface of the backing but have small footprints. Abrading forces apply tipping torques to these tall islands. Thick backings are useful in resisting these torque forces. Also, composite laminated backings are used to provide structural support to these small-surface area (but tall) islands. Increasing the backing thickness and the island height both increase the overall abrasive disk thickness.
D. Heavy-Duty Abrasive Disks
The laminated heavy-duty disks that have raised islands coated with thick layers of abrasive material are thicker than the disks that only have monolayers of abrasive beads. The laminated backings can be constructed of multiple layers of different materials including polymers, metal and fiber mats. These backings can be quite thick. Also, the individual abrasive coated island structures can be substantial in height. The thickness of the disks measured from the island top surfaces to the bottom of the backing is precisely controlled over the whole annular abrasive band.
E. Abrasive Disk Uniform Wear-Down
It is also critical that the abrasive disk is worn-down uniformly across the abrasive surface to maintain the flatness of the disk over its full abrading life. When an abrasive disk wears down uniformly across that full annular area the precision thickness of the disk is maintained. This uniform wear-down of the abrasive is accomplished by matching the width of the disk annular radial width to the flat cross sectional size of the workpiece. Here the full annular width of the abrasive disk is contacted by the workpiece during an abrading operation to assure that abrasive experiences uniform wear.
XIX. Size of Island Disks
A. Typical Disk Size
The disks typically have a 12 inch diameter when small sized workpieces are lapped. The raised island abrasive is located in an annular band where the radial width of the annular band is approximately equal to the diameter (or size) of a workpiece. Large diameter abrasive disks are required for large diameter workpieces. For example, a 300 mm (12 inch) diameter semiconductor workpiece requires an abrasive disk that exceeds 48 inches or 4 feet to provide an annular abrasive band that is 12 inches wide. Having the abrasive disk central 24 inch diameter free of abrasive assures that the abrading surface speed of the abrasive at the inner diameter of the annular band is not substantially different than the abrading surface speed at the outer diameter. The closer the outer and inner diameters of the annular band are to each other, the rotational speed of the workpiece required to even-out the abrading speed across the abrasive annular band is reduced. It is desired to minimize the rotational speed of the workpieces to minimize balancing problems. Un-balanced workpieces rotating at great seeds can cause wobbling which results in non-flat lapped surfaces. It is practical to balance the workpieces which allows them to be rotated at high speeds without wobbling.
Some abrasive disks can be huge. For instance 144 inch (12 feet) diameter disks are the size of a small room. These disks are used to flat lap 300 mm (12 inch) diameter semiconductors.
XX. Heavy Duty Raised Island Disks
A. Disks Replace Micro-Grinding Wheels
Abrasive systems using heavy-duty versions of flexible raised-island abrasive disks can be used to replace the micro-grinding (flat-honing) systems that use rigid metal abrasive-wheels. These heavy-duty flexible abrasive disks are used for aggressive workpiece material removal and for long-life abrading usage. The flexible disks have flat-surfaced raised-islands. Each island has thick layers of abrasive-bead material which allows long term usage of the disk before the disk abrasive wears out. Flexible heavy-duty disks can also have abrasive pellet islands that are attached to durable disk backings. The abrasive coated raised-islands are positioned in array patterns that form annular bands of abrasive around the circumference of the disks.
Quick changing of these heavy-duty disks allows fast set-up changes to be made to the abrading system. Vacuum is used to quickly attach these flexible raised-island disks to rigid flat-surfaced platens. Here, utilization of a wide range of abrasive particle sizes and abrasive particle materials (including diamond, CBN and aluminum oxide) can be made with ease. Rigid micro-grinding abrasive-wheels can not be quickly changed without great difficulty. In addition, the flexible heavy-duty disks are lightweight and easy to handle compared to the massive flat-surfaced heavy metal abrasive-wheels used in micro-grinding.
When changing a micro-grinder abrasive-wheel, localized abrasive surface distortions can occur when the abrasive-wheels are bolted on to platens. These surface distortions originate at the individual mounting-bolt areas and are caused by tightening the mounting bolts. Undesired planar-flatness distortions of only 0.0001 inches can affect the performance of an abrading surface when flat-lapping workpieces. The vacuum hold-down forces of the flexible heavy-duty raised-island disks are spread uniformly across the whole flat surface of the platen and these forces do not distort the platen surface.
Advantages of using these flexible heavy-duty disks include quick-change set-ups, high abrading speeds, low abrading pressures, high productivity, low workpiece polishing costs, great water cooling action, precision-flat and mirror-smooth workpieces (due to the very small particles in the abrasive beads). Because high abrading speeds are used with these heavy-duty raised island disks, the abrading contact forces are just a fraction of those used in micro-grinding. Instead of having “brute-force” workpiece material removal by slow-speed micro-grinding, the high-speed disks provide “delicate-force” abrading contact but also, high material removal rates. These lesser abrading forces result in smaller forces on the island structures. These smaller abrading forces allows flexible (but durable) backings to be used in place of the rigid metal abrasive-wheels used in the micro-grinding. Also, smaller abrading forces result in less subsurface damage of brittle workpieces.
B. Thick Abrasive Layers on Islands
The thick abrasive layers on the island flat top surfaces can be produced by a number of different methods. First, small diameter beads can be mixed with an adhesive and coated on the island tops where many layers of the small beads are stacked on top of each other. Second, very large sized abrasive beads can be coated in monolayers on the island tops. Third, vitrified abrasive island pellets can be adhesively bonded o to a flexible backing. The erodibility of these stacked ceramic abrasive beads is similar to the erodibility of the thick layers of abrasive particles contained in the vitrified abrasive pellets.
C. Abrasive Pellets Attached to Backings
Fused or vitrified flat-surfaced composite abrasive island pellets can be strongly bonded with adhesive to a flexible backing to produce flexible heavy-duty abrasive raised island disks. Open recessed-passageways are provided between each of the pellet island structures. These passageways provide channels for excess coolant water which prevents hydroplaning of the workpieces when the disks are rotated at high abrading speeds. Even though the individual vitrified abrasive pellets are rigid, the backing material located in the recessed areas between the individual island structures is flexible. Because the inter-island backing is flexible, the overall abrasive disk is flexible. Here, the flexible island disks will conform to the flat planar surface of rotary platens, which allows the disks to be robustly attached to the platens with vacuum.
To provide heavy-duty abrasive raised-island pellet disks having a planar abrasive surface that is precisely co-planar with the bottom mounting surface of the backing, special and simple production steps can be taken. First, if the top abrasive surfaces of the pellets are not sealed adequately to hold a vacuum, an adhesive tape can be applied to the top flat surface of each of the individual pellets. Second, the tape-covered pellet islands can be temporarily attached to the flat surface of a first precision-flat platen by vacuum. Individual pellet islands are positioned to have gaps between adjacent islands. They are also arranged to form annular abrasive bands on the platen. Third, a flexible backing can be attached to another precision-flat platen with vacuum. Fourth, an adhesive is applied to the bottom of the exposed surface of the individual pellet bases. Fifth, the first platen holding the pellets is positioned with gap spacers that provide a precision fixed distance from the first platen to the platen holding the backing. As the first pellet platen is lowered to rest on the spacers, the pellet-base adhesive contacts the backing surface. When, the adhesive solidifies, the first platen is then separated from the pellets by interrupting the vacuum, leaving the abrasive pellets attached to the backing with adhesive.
The top flat surface of all of the individual abrasive pellets is now precisely co-planar to the bottom mounting surface of the abrasive disk backing. The adhesive tape (if used) is removed from the pellet island surfaces to expose the pellet abrasive particles. The pellet-type heavy-duty raised-island abrasive disks produced here can be used interchangeably for high speed flat lapping. This is because the disk abrasive surfaces are precisely co-planar with the disk-backing platen-mounting surface. These heavy-duty raised-island abrasive disks have precision thicknesses with very small thickness variations across the whole annular abrading surface of the disk. The absolute thickness of the disks does not have to be constant as just the thickness uniformity is important for high-speed abrading.
D. Vitrified Abrasive Pellet Manufacturing
Both abrasive beads and vitrified pellets provide a porous erodible ceramic support for individual abrasive particles. The abrasive particles are mixed with metal oxide (ceramic precursor) particles and formed into abrasive shapes. Abrasive beads have spherical shapes. Abrasive pellets have flat surfaces with a variety of cross-sectional body shapes. Both the abrasive beads and the abrasive pellets are erodible. When the ceramic matrix material supporting the individual abrasive particles erodes away, worn particles are released and new sharp abrasive particles become exposed to continue the abrading action.
Both the abrasive beads and the vitrified abrasive pellets are processed in high temperature furnaces to convert the metal oxide into a ceramic. Other materials such as metals can also be used along with the metal oxides to produce the abrasive pellets. Modest furnace temperatures are used with the beads to provide a porous erodible ceramic matrix that rigidly supports individual abrasive particles. For vitrified pellet shapes, high furnace temperatures are used to melt the ceramic to form it into a solid glassy state (vitrified) upon cooling. In the pellets, individual abrasive particles are bonded together with strings of the melted and glassy ceramic material. The combination of the ceramic and abrasive particles form the vitrified abrasive pellets.
Because diamond particles break down thermally at high furnace temperatures in the presence of oxygen, the bead furnace temperatures are kept below 500° C. It is necessary to far exceed 500° C. to vitrify (melt or fuse) the ceramic when forming the abrasive pellets. To protect the diamond (pure carbon) particles from reacting with the oxygen and thermally degrading it at these high temperatures, special steps have to be taken. One alternative is to operate the furnace with an inert (non-oxygen) atmosphere, typically with the use of an enclosed retort furnace. This adds to the production expense and increases the complexity of the furnace operation. Another alternative is to plate a thin metal coating layer on the exterior surface of the individual diamond abrasive particles. This metal plating acts as a barrier that prevents high temperature ambient oxygen in the furnace from reacting with the diamond material. This step also adds to the complexity and expense of producing abrasive pellets. The pellets can be constructed with thick layers of fused or vitrified abrasive particles that are attached to inert ceramic island-base materials. These composite abrasive pellets are bonded to the thick and strong but flexible disk backings.
E. Abrasive Disks Less Expensive than Abrasive Wheels
Flexible heavy-duty abrasive disks are much less expensive to produce than heavy micro-grinding flat surfaced rigid metal abrasive coated wheels.
A wide variety of these heavy-duty disks can be stocked by those performing lapping instead of having a large investment in single expensive abrasive-wheels that often are not changed for months of operation. Having the economic freedom to quickly change the type of abrasive or abrasive particles sizes is a huge advantage for those companies that provide lapping services to a wide range of customers.
F. Quick-Change Heavy-Duty Disks
The heavy-duty flexible disks are light weight and easy to handle for quick attachment to the flat platens. Even though the disks have high raised islands and thick backings, the disks are flexible. They also have a continuous and smooth surfaced backing. The flexible and smooth-backside abrasive disks can be quickly attached to flat-surfaced platens with the use of vacuum. The vacuum provides huge attachment forces that “structurally” bond the flexible abrasive disks to the rigid metal platens. These vacuum disk hold-down forces allow the flexible heavy-duty abrasive disks to become an integral part of the rigid platens. Because the disk backings are relatively thin and the islands are rigid, there is very little compressibility of the raised island abrasive disks. The top flat-abrasive surface of the precision-thickness disks automatically becomes co-planar with the precision flat rigid platen surface. Each time an abrasive disk is mounted on the platen, a precision-flat abrading surface is provided for contact with flat-surfaced workpieces.
Because the flexible abrasive disks protect the platen flat disk mounting surface from wear, the precision flat platen surfaces remain flat over long periods of time, even as the abrasive disk surfaces experience wear. The abrasive surface flatness of a disk abrading surface can be quickly reestablished simply by removing a defective disk and replacing it with a new (or previously used) flat-surfaced abrasive disk.
To assure that disks “remember” their abrasive surface planar flatness relative to a given platen, the disks can be marked on the outer periphery. This alignment disk-mark can be registered (aligned) with a corresponding permanent registration mark located on the outer periphery of the platen. The abrasive disk registration marks can be added at the initial installation of the disk on a platen or the disk marks can be incorporated as a feature on new disks. Positioning disks concentric with a platen is easy to accomplish visually because both the disks and the platens typically have the same diameters. Alignment of the disk and platen marks is also easy to accomplish by rotating the disk tangentially by hand prior to application of the disk hold-down vacuum. In this way, any out-of-plane defects of the platen surface are automatically compensated for, after a given disk is dressed-flat on that specific platen surface.
G. Avoid Platen Distortions
It is necessary to attach the heavy and rigid micro-grinding abrasive wheels to platens with threaded fastener bolts. When these bolts are tightened, distortion of the abrasive-wheel is unavoidable in the bolt-hole locations. These rigid-wheel bolt-hole distortions can spread structurally to the planar surface of the wheel abrasive. For high speed abrading, it is critical that the surface of the abrasive have a radial flatness variation of less than 0.0005 inches and a circumferential flatness variation of approximately 0.0005 inches, depending on the diameter of the platen and the platen rotational speed. Otherwise, the non-flat abrasive traveling at more than 10,000 SFM (100 mph) will only contact the workpieces at the abrasive “high-spot” areas. This non-flat abrading contact is highly undesirable. A reverse-analogy here is an auto traveling at high speeds over a washboard road (high-spot abrasive areas). The auto will be “floated-upward” by the continual excitation of the periodic bumps of the washboard road surface. Controlled stability of the auto is lost until the auto reaches a smooth road surface. It is preferred that the platen has a radial flatness of standard deviation of less than 0.0002 inches and a circumferential flatness variation standard deviation of slightly greater than 0.0002 inches. It is more preferred that both the radial and circumferential flatness standard deviation is less than 0.0001 inches
The micro-grinding non-flat abrasive surfaces have to be abrasively conditioned after an abrasive-wheel is changed. This conditioning removes the high spots from the wheel abrasive surface. When the abrasive wheels are changed on a micro-grinding system, it is a long and laborious procedure. Thee rigid wheels are heavy and difficult to handle manually.
Great care has to be exercised in tightening the wheel hold-down bolts so that the whole wheel body is joined to the platen body without distortion to the wheel body. This procedure is analogous to the careful bolt-tightening pattern procedures required for attaching the valve-head to the block of an automotive engine without distorting the head.
Part of the motivation to provide such thick abrasive layers on the abrasive wheels is the great difficulties present in changing the rigid abrasive wheels. None of these abrasive surface distortion concerns are present when a new flexible heavy-duty abrasive disk is attached to the surface of a flat platen. Here, the lightweight disk is simply laid by hand on the surface of the platen and vacuum is applied. The disk-attachment hold-down vacuum immediately bonds the disk to the rigid and precision-flat platen surface. The flexible disk becomes an integral part of the rigid and strong platen.
Because the vacuum attachment forces act uniformly across the full surface of the disk there are no localized distortion applied either to the platen or to the disks. This allows the heavy-duty flexible abrasive disks to be mounted repetitively on the platens. Each time a flexible disk is re-mounted on a platen, the abrasive regains its original precision planar abrasive surface that was already established with earlier use on the same platen. First-time conditioning-use of a raised island disk compensates for any out-of-plane flatness variations on the platen surface. These re-mounted disks can be used immediately to successfully abrade workpieces at the desired high abrading speeds.
H. Reduced Subsurface Damage
The small abrading forces used in high-speed abrading with the heavy-duty flexible disks results in less subsurface damage of brittle workpieces than occurs with the high abrading force micro-grinding systems.
I. Heavy-Duty Disk Platens
The platens used with these heavy-duty raised island abrasive disks have a structurally and dimensionally stable construction so they remain precisely flat over long periods of time.
XXI. Workpiece Cooling with Islands
A. Coolant Used to Avoid Thermal Cracks
Sufficient water is applied to the workpiece and abrasive to provide surface cooling under the whole flat surface of the lapped workpiece. This water is used to remove the friction heat that was generated by the abrading action of the moving island abrasive. This friction heat can damage both the workpiece and the individual diamond abrasive particles. It is desirable to quickly remove the heat from a localized workpiece abraded area before it has a chance to “soak” into the depths of the workpiece. If a localized area of a workpiece is heated, the thermal expansion of the heated area tends to cause thermal stresses in the workpiece material. Ceramic materials are particularly susceptible to the thermal stress which can cause undesirable localized stress cracks.
B. Islands Carry and Spread Coolant Water
Water that is applied to the leading edge of a workpiece minimizes the coolant water velocity as it travels along with the high speed abrasive. This “stationary” water tends not to be driven into the wedge areas of the leading edge of the workpiece. Water that is applied to a continuous-coated abrasive surface upstream of the leading edge and moves at high speeds is driven into these wedges and causes hydroplaning or lifting of the workpiece.
Raised islands that contact the “stationary” bead of coolant water at the workpiece leading edge tends to “chew-off” a portion of water and push this portion along the flat workpiece abraded surface. The water clings to the flat abraded side of the workpiece rather than falling away from the surface. This clinging is due to surface tension and other liquid adhesion forces. Also, the curved or angled leading edge of the island “snowplows” the water portion off to the island-travel pathway sides as the island travels under the workpiece. The snowplowed water wakes wet the surface of the workpiece that had been abraded by a previous island that had preceded it on an adjacent travel-pathway. In this way, the coolant water is constantly spread or washed across the surface of the abraded workpiece surface by the island structures. Because the islands travel at such high speeds, the water coolant effects take place immediately after the friction heat was generated on the workpiece surface by a preceding abrasive island.
In addition, the coolant water has special heat transfer characteristics for cooling the cutting tips of diamond abrasive particles that can be heated to very temperatures by this friction heating. When the diamond cutting edges are heated to more than 212 F, the diamond edge-contacting coolant water vaporizes and provides huge cooling to the diamond due to the localized vaporization of the water. The high associated coefficients of heat transfer with this water-boiling effect maintains the diamond edge temperatures to much less than that which will degrade the sharp cutting edges of the individual diamond particles. Any steam produced is routed to the recessed channels between the raised islands which prevents the steam from lifting the workpiece away from the flat abrasive surface.
Double-Sided Floating Platen Systems
Double-sided slurry or micro-grinding (flat-honing) systems also use the approach where the upper floating platens contact equal-thickness workpieces. However, the workpieces are not independently supported by multiple rigid fixed-position spindle surfaces that are co-planar. Rather, both the floating double-sided upper platen and the rigid-supported lower abrasive platen are independently rotated with equal-thickness workpieces sandwiched between the two platens. Multiple flat-surfaced workpieces are spaced around the annular circumference of the lower platen and they are held in abrading contact with the lower platen abrading by the upper abrasive platen. Both opposed surfaces of the workpieces are simultaneously abraded by the concentric rotation of both the upper and lower platens. Workpieces are rotated during the abrading action to provide uniform wear on the workpiece surfaces even though the abrading speeds, and the corresponding workpiece material removal rates, are different at the inner and outer radii of the platen annular abrasive bands.
Both the upper and lower platen abrasive surfaces are continuously worn into non-planar conditions by abrading contact with the abraded workpieces sandwiched between them. In double-sided floating-platen abrading, the workpieces are held by gear-driven planetary workholder carrier disks that rotate the workpieces during the abrading action. These carrier disks must be thinner than the workpiece to avoid abrading contact of the carriers with the abrasive on both platens. Abrading forces are applied to these thin carriers by the rotating platen abrasive surfaces and portions of these abrading forces are also applied to the planetary carrier drive gears. These thin and fragile workpiece carriers, that are also sandwiched between the platens, can not be driven at high speeds by the carrier disk drive gears. Because of limitations of the workpiece carrier system, both double-sided slurry lapping and micro-grinding (flat-honing) systems operate at low abrading speeds. Double-sided slurry lapping typically has low abrading pressures but double-sided micro-grinding (flat honing) utilizes very high abrading pressures. The workpiece abrading pressures are applied by the upper platen. Because the workpiece abrading pressures of the double-sided micro-grinding (flat honing) system utilizes very high abrading pressures, the upper and lower platens must be strong enough to resists these pressures without distorting the platen planar abrading surfaces. As a result, these platens are typically very heavy in order to provide the required structurally stiff platen abrasive surfaces. Use of very heavy upper platens results in difficulty in accurately controlling the low workpiece abrading pressures desired for high speed flat-lapping.
CMP-Type Floating Spindle Systems
Some CMP abrading systems use multiple workpiece spindles that are attached to a common frame that is suspended above a flat-surfaced rotating platen. The platen is covered with a resilient abrasive pad. Wafers are attached to the individual spindles and then the frame is lowered to bring all of the individual spindle-rotated wafers into abrading contact with the pad as the pad is rotated by the platen. The dimensional amount that each wafer is plunged into the surface of the thin liquid abrasive slurry-coated resilient pad is not precisely controlled. Instead, the abrading force that the individual wafers are pressed into the pad is typically controlled by the mechanisms that apply forces to the individual spindles. Penetration of the flat-surfaced wafer body into the pad surfaces also varies by the localized stiffness of the resilient pad. This pad stiffness changes during the CMP process as the abrasive slurry builds up a crusty solidified deposit coating on the pad. This crusty surface is broken up periodically by use of an abrasive-particle coated conditioning ring that is held in force contact with the moving pad.
There is no critical precision static or dynamic machine component co-planar surface requirement present for these CMP platens because the floating individual wafers are forced into the surface-depths of the resilient abrasive pad as the pad is rotated. Likewise, there is no critical requirement for the alignment of the flat abraded-surfaces of each of the individual spindle-mounted wafer to be located precisely in a common plane. This lack of co-planar alignment criteria occurs partially because of the wide positional tolerance of the wafer spindles allowed by penetration of the wafer surface into the surface-depth of the pad. Further, there is no requirement that the surfaces of the individual wafers be precisely co-planar with the flat surface of the rotating platen, again partially because of the wide wafer surface positional tolerance allowed by penetration of the wafer surface into the surface-depth of the pad. These co-planar spindle surface alignments are not necessary because each of the spindles is independently moved along its rotation axis. By simply controlling the applied abrading pressure at each workpiece spindle, the spindles are allowed to move freely along their rotation axes to provide the desired abrading pressure, independent of the movement of the other adjacent workpiece spindles.
There is a distinct difference in the technologies used by the floating-spindle CMP abrading system and the fixed-spindle floating-platen abrading system. The CMP abrading system is a distributed-spindle pressure-controlled axial-motion workpiece spindle system. It is not a rigid non-movement workpiece spindle system like the fixed-spindle-floating platen abrading system. This CMP abrading system can not perform the precision workpiece abrading functions that the fixed-spindle-floating platen abrading system can because the CMP system does not have the precision fixed-position rigid planar surface abrading system. CMP abrading consists only of removing a layer of material from an already-flat workpiece surface by polishing action. It does not establish a planar flat surface on a workpiece. Rather, it just provides a surface-polishing action. However, the fixed-spindle-floating platen abrading system can establish a planar flat surface on a workpiece even if the workpiece has a non-flat surface when the abrading action is initiated. Both systems use rigid platens. The CMP platen is rigid but the flat abrasive pad that is attached to the rigid platen is resilient. The CMP workpieces are not in abrading contact with a rigid abrasive surface; the workpieces are in abrading contact with a resilient pad abrasive surface.
Slurry Lapping
Conventional liquid abrasive slurry can also be used with this fixed-spindle floating-platen abrading system by attaching a disposable flat-surfaced metal, or non-metal, plate to the rigid platen surface and applying a coating of a liquid loose-abrasive slurry to the exposed flat surface of the plate. The platen slurry plate can be periodically re-conditioned by attaching equal-thickness abrasive disks to the rotating workpiece spindles and holding the rotating platen in abrading contact with the spindle abrasive disks. Here again, the primary planar reference surface even for the platen is the granite surface planar surface.
There are still many improvements in this area of technology that can be made according to practices and enabling apparatus, systems and methods described herein. All references cited in this specification are incorporated by reference in their entirety.